Laminaria hyperborea is a perennial and lives for up to 20 years. Longevity is thought to be higher in its northern distribution (Sjøtun et al., 1993).

Spores are produced from sori over most of the blade surface (except most distal or proximal areas) over 6-7 weeks in winter (September - April) (Kain, 1975). Most young sporophytes (germlings) appear in spring but can appear all year round depending on conditions (Birkett et al., 1998b).

The obvious plant is the sporophyte (diploid) producing vast numbers of meiotic haploid zoospores from 'sori'.

The flagellated zoospores are about 5 microns in diameter (Sauvageau, 1918; cited in Kain, 1979) and may be transported at least 5 km from the parent (Jónsson, 1972, cited in Norton, 1992). They lose their flagella after 24 hrs (Kain, 1964) and settle on the available substrata. However, settling rate is dependant on the local currents, therefore larval settling time is probably longer than 1 day (Fredriksen et al., 1995).

The zoospores develop into microscopic dioecious gametophytes. These become fertile in 10 days in optimal conditions.

Male gametophytes release motile sperm that fertilize eggs of female gametophytes and the resultant zygote develops into the new sporophyte. Mass and rapid sperm release was initiated by adding a drop of sea water, into which female Laminaria hyperborea gametophytes had released eggs, to the male gametophyte culture medium, suggesting the eggs produce pheromones which induce the release of and attract the sperm (Lüning & Müller, 1978).

Maturation of the gametophytes can be delayed under less optimal conditions and development remains vegetative. For example, Lüning (1980) reported that fertility, the induction of fertilization in male and female gametophytes, depended on a critical quantum dose of blue light. Fragments of damaged vegetative gametophytes may develop into separate gametophytes (only a few cells are required) hence reproductive potential may be increased. If optimal conditions return the gametophyte may become fertile and produce gametes.

Spore production may be inhibited by epifauna such as Membranipora membranacea (sea mat) and endophytes such as Streblonema sp. (Kain, 1975b).